Plastic chalkboard



Sept. 12, 1950 c. B. EGOLFY 2,522,351

, 'PLASTAIC CHALKBOARD Filed Aug. 15, 1949 Layer of Abrasive Rig/d Plasll'a Material Mal/e Surface IN VEN TOR.

CHARLES E. EGOL F ATTORNEY Patented Sept. 12,1950 r I Charles B. Egolf,.lP hiladelphia, Pa., assignor to Rohm & Haas Company, Philadelphia, Pa. a

corporation of Delaware v Application August 15, 1949, Serial No. 110,478 "4 Claims. (01. 235-68) This invention relates plastic chalkboard and particularly to an. acrylic resin chalkboard that has been made abrasionresistant by the incorporation of hard abrasive materials in the writing surface.

It is known that sheets of styrene or of acrylic resins, such as polymerized methyl methacrylate and copolymers of methyl, methacrylate with other polymeriza-ble materials, may be formed by to anabrasion-resistant h are particularly satisfactory. The methyl methacrylate may be-the sole polymerizable material or it may be polymerized in admixture with other polymerizable materials, particularly the methyl and ethyl esters of acrylic acid. Polystyrene plastics are also suitable, particularly the polystyrene plastics that are modified with acrylic derivatives such as methyl acrylate, ethyl acrylate and acrylonitrile or with linea polyesters of glycols and polymerizing the monomeric materials under con- 9;;dibasic acids at least partially composed of maleic trolled conditions in cells formed by sheets of or fumaric acid. These plastic materials are all glass that are sealed about theirouteredges with known in the art. a flexible material. Such processes" are described The abrasive material is preferably carborunir'i'U. S. Patents 2,154,639 and 2,328,525Q1n these, dum powder or silica. The size of the abrasive processes, the surface of the glas's'sheet against pa.rticles may vary from 5 to 150 microns. A which the polymer is formed is reproduced in the diameter of less than 100 microns is preferred sheet of polymer with any design in reverse. By because premature settling during the mold-fillpolymerizing the monomeric materials in "a cell ing operation does not occur with particles of that made from a sheet of glass having a matte sur size. Very small particles should be avoided as face a sheet of polymer is obtained Which=als0 9..- they require too long a settling time and do not possesses a matte surface. Such a surface on a give satisfactory results. The abrasive material plastic sheet forms a very satisfactory surface for should constitute 0.1% to by weight of the writing with chalk but on repeated use the surface mixture to be formed into a sheet. acquires a glare and becomes unsatisfactory. The single figure of the attached drawing is It has now been found that the incorporation an end view, partially in cross-section, of a chalkof particles of abrasive materials in or closely board within the scope of the present invention. adjacent the-matte surface of the plastic sheets As will be noted therefrom, the chalkboard comnot only greatly improves the wear-resistance of prises a sheet of rigid polymeric plastic having the matte surface but gives a product which ima matte surface with particles of abrasive extendproves with use. To accomplish this, particles ing in a layer of substantial depth adjacent said of the abrasive are added to the monomeric matematte surface. rial which may or may not contain dissolved poly- The following example is given to illustrate mer to increase the viscosity of the mixture. This the preparation of the plastic chalkboard: mix is then charged to a cell formed of two sheets A cell was prepared from two lights of of glass held apart and sealed at their edges, one x x 1/ H v of the sheets having an inner matte surface, A 4

suitable matte surface on the glass sheet may be plate glass, one Of which had a Sa dla ted 511 obtained by sand-blasting or honing. During the These lights were held apart y Small P yfllling of the cell, it should be in an almost horiyl m h ryl e pyramids 0.3" high and the zontal position with the matte surface at the 40 edges Of the cell were formed o s ps of summed bottom. The filled mold is allowed to remain in P e e d around the perimeter f t a horizontal position until the particles of abra- Sheets- The Sand-blasted ass Surface faced insive settle to form a layer adjacent the bottom Ward nd an Opening was left in the edge of the of the matte surface of the polymeric sheet. The 0911 for The 0811 Was placed in Substanmaterial in the cell is then brought to the fully tiallv i on al p itio with e da polymerized state in the conventional manner. light on the bottom and the Opening Slightly Pigments or dyes may be added to the monoraisedmeric material to give a polymeric sheet having A mixture of 600 grams of yl ethacrylate a color that contrasts with the. color of the chalk monomer containing ben oyl peroxide; to be used. A phthalocyanine blue or green shade 5 stearic acid and 0.075% of a phthalocy nine is particularly pleasing, pigment was heated With agitation t0 80-100 C.

The polymeric plastic material may be formed until dissolved air was expelled and p ymerizafrom any monomeric material which on polytion had advanced to the extent of 5% solids. merizing in sheet form results in rigid sheets. The Itv was then cooled and 12 grams of 200 mesh acrylic plastics based upon methyl methacrylate silica was added with stirring. The suspension horizontal position with the sand blas'ted lights.

on the bottom and air at a temperature of 70 C. was circulated about them until polymerization was complete. The glass plates were then removed from the plastic sheets and the latter heattreated at Bil-100 C. to relieve any strains. step is described in U. S. Patent 2,157,049.

In each case there resulted a plastic sheet having a matte surface well adapted for use as a chalkboard. To determine themermanence of the matte surfaces, each sheet was abraded for 1000 cycles by rubbing with a felt eraser and chalk dust and the percentage of light reflectance measured. The'refleotance was measured photometrically by allowing light to strike the surface at an angle of 45 and the reflectance measured at an angle of 45. The reflectance of a vitreous enameled standard was taken as 100%. The readings were 37 for the sample containing no silica and 9% for thesample containing the silica. This indicated the effect of the silica in preventingthe development of glare.

Analysis of the sheet containing the silica showed that the abrasive had settled and was concentrated in the section adjacent the matte surface.

The same procedure can be followed in making plastic sheets suitable for use as chalkboards from the other polymerizable materials heretoforementio'ned. The polymerization procedure may also be varied in ways well known in the art of'making sheets from such polymerizable materials". 1

I claim: 5 v 1.'A chalkboard comprising a sheetof rigid This file of this. patent:

polymeric plastic having a matte surface and having dispersed in the plastic and concentrated adjacent the matte surface hard abrasive material of particle size from 5 to microns, said abrasive material amounting to from about 0.1% to about 20% of the total weight of the sheet.

2. A chalkboard comprising a sheet of rigid polymeric plastiqicontaining a preponderance of methyl inetl'iacrylate having a matte surface and having dispersed in the plastic and concentrated adjacent the matte surface hard abrasive material. of 'particle size from 5 to 150 microns, said abrasive material amounting to from about 0.1% to about 20% of the total weight of the sheet.

3. A chalkboardcomprising a sheet of rigid polymeric plastic containing a preponderance of methyl methacrylate having a matte surface and having dispersed in the plastic and concentrated adjacent the matte surface particles of silica of size from 5 to 150 microns, said silica amounting to fromabout 0.1% to about 20% of the total weight of the sheet.

4. A chalkboard comprising a sheet of poly-' methylmethacrylate having a matte surface andhaving dispersed and concentrated in the portion adjacent the matte surface particles of silica UNITEDi STATES PATENTS the 

